What!? The crab you ate was actually a fake crab?

When it comes to crabs, I believe everyone is familiar with them.

A pair of large pincers with a round and flat carapace

Plus symmetrical crab legs

This is what crabs look like to us

Although seeing is believing, there are actually quite a few "ghosts" among crabs. There is a group of crustaceans that secretly imitate the appearance of "crabs" and blend into the group of "real crabs". Therefore, many of the crabs that are common or often eaten in our lives are "fake crabs."

For example, the well-known king crab and coconut crab are actually pretending to be “crabs” !

Figure 2 Alaskan king crab source network

0 1 Real crabs and "fake crabs"

Judging from the appearance, the "fake crab" can achieve such a realistic imitation. No one would believe that it has nothing to do with the "real crab".

In fact, whether it is a "false crab" or a "real crab", they all belong to the large family of Decapoda . The so-called "decapoda" refers to five pairs of walking legs. Let's look at crabs to quickly understand it. There is a pair of crab claws plus four pairs of crab legs, exactly five pairs.

Figure 3: Counting crab legs. Source: Bhny

Seeing this, careful friends may have realized that there is another type of animal that also has five pairs of legs. Yes, it is shrimp. The common shrimp, crayfish, lobster, etc. also belong to the order Decapoda .

Image 4 Source: Bart Braun

As for the relationship between shrimps and crabs, it is not as philosophical as that between eggs and chickens, because according to current fossil research, the earliest known decapods are lobster-like decapods, which lived in the Devonian period 360 million years ago.

About 100 million years later, a branch of the Decapoda split into two groups, including Brachyura and Anomura.

In the years that followed, these two groups, which originally belonged to lobsters, went further and further down the path of becoming crabs. They gradually shortened their long shrimp tails a little bit and hid them under their abdomens, just like humans gradually discarding their tails.

Figure 5 Source: joaana

At the same time, their body shape began to develop horizontally, from a round shape to a round and flat shape, and finally became the crab shape we see today. This process is called "crabization" by biologists, also known as short-tailization.

Among them, the Brachyura are very specialized in being crabs. They completely abandon the shrimp tail and flatten the tail and tuck it under the abdomen until the entire body is covered with carapace. Biologically, they have a standard crab appearance and are also called true crabs.

Image 6 Source: fiddlercrab

However, some crabs of the order Heterocura can actually be seen with shrimp tails, such as the hermit crab. If you take it out of its "house", you can clearly see a section of the tail. Therefore, although it is also called a crab and even looks very similar to a crab, its spiral abdomen and fragile exoskeleton reveal that it is not a "real crab."

Look carefully at Figure 7. When the hermit crab changes its home, there is still a long section of its tail. Source: see watermark

This is also an important feature to distinguish "real crabs" from "fake crabs".

In addition, we can quickly distinguish them by the number of their legs. Simply put, real crabs can clearly see four pairs of crab legs, while fake crabs can only see three pairs of legs.

Someone must be asking here, are pseudo crabs missing a pair of legs? Can they still stay in the decapod family?

In fact, although it cannot be seen on the surface, the fourth pair of legs of the false crab still exists, but they are very short and cannot help with walking, so they are hidden.

Figure 8 Porcelain crab, from the outside it looks like it only has three pairs of legs. Image source: yammo

0 2Why does everyone want to be a crab?

Although real crabs and "fake crabs" embarked on different evolutionary paths many years ago, their final forms are the same. They all chose to explore the world in the form of crabs . It's a bit like the saying "all roads lead to Rome."

In biology, this phenomenon is called convergent evolution , which means that different groups independently evolve similar characteristics . This is common in the evolution of many animals. For example, we can find traces of birds in bats, and we can also see a hint of "dog" in your good friends (bushi).

But what’s interesting is that the crab-like body structure has been “ruined” in this group of crustaceans.

According to research statistics, over the long years, "crabization" did not occur in one go, but rather underwent at least five evolutions. In this process, there was even a "crab-de-crab" phenomenon. That is to say, for a period of time, some groups gave up becoming crabs, but later returned to the path of becoming crabs. It's very strange.

So why do people want to be a crab? Is it because lobsters are not delicious?

On the contrary, it is because lobsters are so delicious, especially the lobster tail. This part is key to the lobster's movement in the water, so it has developed a lot of muscles , so the taste is naturally good.

But the problem is that the lobster tail, which looks delicious, is only protected by a fragile exoskeleton. For predators, the lobster's twisting tail is tantamount to a walking temptation.

In order to make themselves look unpalatable, some lobsters began to shorten their tails or even hide them in their abdomens. Their cylindrical bodies also became flatter, and finally they became crabs covered with shells and without much meat , in exchange for more chances of survival.

In addition to making themselves look unpalatable, the crab-like body also provides a larger living space. Compared with the cylindrical body of the lobster, the flat shape of the crab can better help them move sideways, greatly improving their flexibility .

Picture 11 is gone. Image source: quora

Moreover, in addition to evolving for walking, crabs' legs also have the ability to dig under sediments or paddle in the water , allowing crabs to easily adapt to new habitats.

Currently, there are more than 7,000 species of Brachyura in the world, but there are only a handful of lobsters. It is not difficult to see that the crab-like body structure is indeed outstanding.

Up to now, "crabization" has not stopped. In the future, perhaps more crustaceans will choose to become crabs. In this process, their mental journey may be to question crabs, understand crabs, and become crabs!

References:

Why Do Animals Keep Evolving into Crabs?

https://www.scientificamerican.com/article/why-do-animals-keep-evolving-into-crabs/

How to become a crab: Phenotypic constraints on a recurring body plan

https://onlinelibrary.wiley.com/doi/10.1002/bies.202100020

Wikipedia—Carcinisation

https://en.wikipedia.org/wiki/Carcinisation

Author: Fish

Winner of the Silver Award for Outstanding Science Popularization Works of China Science Writers Association

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Editor: Dong Xiaoxian